X-ray absorption spectroscopy at the molybdenum K-edge has been used to probe the molybdenum coordination of Rhodobacter sphaeroides dimethylsulfoxide reductase. The molybdenum site of the oxidized protein possesses a novel Mo(VI) mono-oxo site (Mo=O at 1.68 w) with additional coordination by approximately four thiolate ligands at 2.44 w and probably one oxygen or nitrogen at 1.92 w. The reduced Mo(IV) form of the enzyme is a des-oxo molybdenum with 3-4 thiolates at 2.33 w and two different Mo_O/N ligands at 2.16 w and 1.92 w. Similarly, the stable Mo(V) glycerol-inhibited species is found to be a des-oxo molybdenum with approximately four thiolate ligands at 2.40 w and (probably) two similarly coordinated oxygen or nitrogen ligands at 1.96 w.